Extracorporeal blood oxygenators are medical equipments used in surgical operations like open heart surgery where lungs cannot perform their usual gas exchange function of adding oxygen to the blood and removing carbon dioxide from the blood. In 1953 the Gibbon oxygenator was used with clinical success (1). It consisted of an ensemble of vertical metallic screens where the blood circulated adjacently in the downward direction. This ensemble was inserted in a chamber that allowed oxygen to be transferred to the blood and carbon dioxide to be removed from it (1).
In the next two decades this simple equipment triggered numerous developments in two main directions:                1—The bubble oxygenators—these involved the direct bubbling of oxygen through the blood with the diffusion of the oxygen from the bubbles into the bloodstream and in the opposite direction the diffusion of carbon dioxide from the blood into the bubbles.        2—The membrane oxygenators—they incorporated a semipermeable membrane that was interposed between the bloodstream and the gas phase and therefore allowing at a controlled rate the permeation of oxygen into the blood and the passage of carbon dioxide from the blood into the oxygen stream.        
In the first ones, the subsequent separation of bubbles and foams was necessary as they presented the risk of damaging the red blood cells. In association with this drawback their use was limited to surgeries involving short times of a few hours. Despite that the use of bubble oxygenators continued till 1980 when they were to a great extent substituted by the membrane oxygenators.
These allowed the support of patients during longer surgery times. They are currently designated by the abbreviation “ECMO” (extracorporeal membrane oxygenator).
Although the ECMO represented a great advancement in decreasing the risks of blood damage and allowing longer times of surgery support they were in the years 1980 still associated to several problems:                the fluxes of oxygen transfer where still lower than the bubble oxygenators;        the costs where high;        there were higher risks of failure due to leakages;        they required higher blood volumes.        
These problems have successfully being solved, firstly with the integration of the membrane module with the reservoir of venous blood and then with the incorporation of the improvements made on the manufacture of other extracorporeal equipments like the ones of hemodialysis were leakages where controlled, robustness improved and costs decreased.
However the challenges in the development of membrane blood oxygenators are not only posed at the level of the membrane configurations of flat sheet and hollow fibers modules but mainly at the level of development of new membranes associating properties of hemocompability and higher oxygen permeation rates.
The hollow fiber modules of most of the ECMO in use are frequently made of microporous membranes due to the fact that they are characterized by high gas permeation rates.
However despite the fact that the pores of the microporous membranes are of sufficiently reduced sizes to avoid the passage of the blood components like the red blood cells or the platelets, they are prone to severe fouling caused by adsorption of plasma proteins that leads to the activation of the coagulation cascade, formation of trombin, platelet adhesion and formation of thrombi. This means therefore that these membranes do not have hemocompability. In very general terms a membrane that is hemocompatible does not introduce by contact with blood in circulation any alteration of the blood and particularly does not induce hemolysis, thrombogenicity and platelet adhesion through the activation of the coagulation cascade.
In accordance with British patent GB 1595058 another problem associated with microporous membranes is the passage of water to the gas compartment and the dissolution of carbon dioxide and subsequent decrease of concentration gradients and of the gas permeation rates to values below the average value in the human lungs. The authors of the patent GB 1595058 use a pulsed flux to maintain higher mass transfer rates.
One form to overcome these problems is through the coating of the membranes or through the use of composite membranes as described in U.S. Pat. No. 4,622,206 and US 2002/002815611 respectively.
The patent EP1043035A1 mentions another problem connected to microporous membranes which is related to the fact that along the operation time there is occurrence of the intrusion of plasma to the oxygen side and this renders the oxygenator inoperational. The author of this patent EP 1043035A1 refers that this problem can be minimized through the use of a coating that although does not cover some of the very small pores it delays the period of time that the intrusion of plasma can lead to the non-operation of the oxygenator.
Janvier et al. (3) mention that in a membrane of vinyl chloride having local roughness areas of the order of 9 μm, the platelet adhesion was multiplied by a factor of 3 when compared to a membrane with a smooth external surface.
Homogeneous dense membranes either in the form of flat sheets or hollow fibers, may play an important role in the improvement of hemocompability due to the fact that in contrast with the microporous membranes, they display a surface morphology that assures a smoother membrane/blood interface. However, the dense membranes usually present low gas permeation rates that can be lower than the physiological levels required and therefore to meet the necessary oxygenation rate, the membrane surface area of the oxygenator should be increased. And this larger blood/membrane interface may lead to higher risks of blood damage.
Also, the development of non-thrombogenic polymers like the ones reported in the U.S. Pat. Nos. 5,004,461 and 4,521,564 can only be an asset in the solution of these problems if they prove to have film-forming properties and enable the formation of membranes with adequate flux permeate rates and with surface morphologies that associate adequate properties of hemocompatibility.
At present, the clinical use of extracorporeal membrane blood oxygenators and hemodyalizers is always associated to administration of heparin to the patient. This assures that the contact of blood with the membranes and with the equipments does not trigger thrombogenic effects, platelet adhesion and activation of the coagulation cascade. Although there are also reports claiming the minimization of these effects through the additional coating with heparin of the membranes and of the equipment surfaces, Janvier G. et al. (3) refers on the contrary, the occurrence of negative interactions with the systemic heparin.